1. Field of the Invention
The present invention relates to a semiconductor laser device equipped with a current non-injection region near a resonator end face, a fabrication method thereof, and a solid-state laser apparatus equipped with the semiconductor laser device.
2. Description of the Related Art
In semiconductor laser devices, the current density at the end face is increased under high-output emission, so that non-emission recombinant current increases and end-face destruction, etc., occur. Because of this, it is difficult to obtain high device reliability.
To solve the problem mentioned above, a variety of structures for forming a current non-injection region near the resonator end face have been proposed. For example, there is a semiconductor laser device which has a current non-injection region by not forming an electrode near the end portion of the contact layer (IEEE Phonics Technology Letters, vol. 12, No. 1, January 2000, p 13-15). However, in this structure, the current non-injection at the device end face is incomplete because spreading of current through the contact layer occurs near the end face. Japanese Unexamined Patent Publication No. 11(1999)-354880 discloses another semiconductor laser device. In this semiconductor laser device, an etch-stopping layer and a contact layer are stacked on a cladding layer, and a portion of the contact layer near the end face is removed until the etch-stopping layer is exposed. An electrode extending to the end face is formed on the etch-stopping layer. Since the electrode near the end face is in contact with the etch-stopping layer, the current non-injection at the end face is incomplete. Japanese Patent Application No. 2000-311405 (filed by the present applicant) discloses a ridge-structure semiconductor laser device having a current non-injection structure near a resonator end face, and a fabrication method thereof. Japanese Patent Application No. 2000-253518 discloses a current-constriction-structure semiconductor laser device having a current non-injection structure near a resonator end face, and a fabrication method thereof.
On the other hand, a multiplicity of structures have been proposed to make the bandgap energy at a resonator end face larger so that emitted light is not absorbed. For instance, a semiconductor laser device with InGaAsP tensile strain barrier layers is disclosed in Japanese Unexamined Patent Publication No. 11(1999)-220224 and JJAP, vol. 38, pp. L387-389, No. 4A, 1999. The InGaAsP tensile strain barrier layers are formed on the top and bottom surfaces of an InGaAs active layer. With this structure, lattice relaxation occurs in the active layer near the light-exit end face, the bandgap is increased, and the light absorption at the light-exit end face is reduced. This reduces reactive current that can be the cause of heat generation.
In the case where the aforementioned current non-injection region is formed in a semiconductor laser device having a ridge structure, the step of forming grooves and the step of removing a portion of the contact layer to form a current non-injection region must be performed, which is troublesome and time-consuming. In addition, the photoresist material, applied by spin coating, is formed thicker in the etching grooves than on the flat portions and is hardened by an NH3:H2O2 mixed water solution that selectively etches the contact layer. Due to this, when photo-lithographic etching is to be performed again after formation of the ridge grooves, the photoresist lift-off performance deteriorates and some photoresist material remains in the grooves. If photoresist material remains, there is a problem that the unremoved photoresist will contaminate the grooves, thereby reducing the adhesion of the insulating film thereto and that a hollow, etc., will be formed in the subsequent electrode-sinter heat treatment step, etc., which will reduce heat radiation performance during laser emission. Furthermore, there is a problem that after formation of the grooves, the side face of a layer exposed to the groove for a long period of time will be oxidized, increasing crystal defects, which stops laser emission. Particularly, in the case where the layer exposed to the groove is composed of AlGaAs, it is liable to be oxidized.
Hence, it is conceivable that the order of the ridge forming step and the contact-layer removing step is reversed and that the contact layer is first removed and then the grooves are formed. However, after a portion of the contact layer near the end face is removed, the cladding layer underneath the contact layer is oxidized. Because of this, there is a problem that reproducibility of the depth of groove etching will not be satisfactory. Since the groove depth determines an equivalent refractive index step difference ΔNeff related to the profile of laser light, it is undesirable to adopt a process where reproducibility is not obtained. In addition, the aforementioned barrier layer having tensile strain is not sufficient to enhance reliability under a higher output.